The present invention relates to a satellite communication system, in particular, relates to such a system using a stationary satellite with double hops transmission.
Conventionally, a satellite communication system has a single satellite, which illuminates a plurality of earth stations, and the communication between earth stations is performed directly through the satellite.
However, a small earth station using a small antenna is impossible in a prior art, except for some special cases like a mobile communication and/or a boat communication, because a small antenna has a relatively high side lobe radiation, which interferes with adjacent satellites. An antenna with the diameter 1-3 m is not enough for practical use in view of side lobe radiation.
One of the proposals for solving the above problems is a satellite communication system in which a modulated signal is spread over a wide bandwidth (SS modulation) to reduce the power density. However, when a plurality of small earth stations use the same carrier frequency, the total interference power may be still high, and therefore, the problem is not solved essentially.
An excellent antenna which has little side lobe radiation has been developed lately. An offset antenna is an example of that antenna. However, a further problem arises when the low side lobe small antenna is used. That problem is the low C/N (carrier to noise ratio) because of the low antenna gain due to small size of the antenna.
Therefore, a satellite communication between two small earth stations is difficult in the prior art.